I've already explained how you can set-up your watch using RSS and Google Reader.
It is now possible to watch any knid of web site page, even if this given page of a web site does change, Googler Reader informs you that the page has changed.
As an example, let me subscribe to a competitor home page to see if it changes over time :
First click the Add a subscription button and enter the site URL such as :
Create a feed button, you will get this result:
You now can categorize this feed to the Competitors folder using the Feed settings... button:
You are done !